[sidney]

According to JFV 10/III, Oblt. Wübke was the Staffelkapitaen of 7./JG 54 (i.e., in the west) until at least 31 December 1943.

I see a couple of possibilities here. Firstly, the writing under the cockpit is hard to read, so it might not be "Im Auftrage der Reichsbahn" after all, but only similar in style.

Secondly - the aircraft might not have belonged to I./JG 54, which at that time, supposedly the spring of 1944, was equipped with the Fw 190 A aircraft, but some other unit. If the plane indeed belonged to JG 54, one possible candidate could be IV./JG 54, which until at least until 31 December 1943 did not introduce a Gruppenzeichen on the aft fuselage of its aircraft.

Regards,
Sinisa